Method for making foundry moulds

ABSTRACT

Method for continuous, high speed production of foundry mould parts by pressing them from a continuous bed of foundry sand mix which is based on a curable binder, pressing of the part being carried out at a time when the mix has adequate plastic flowability for pressure moulding and adequate potential curability to assure a strong, rigid cured product.

United States Patent 1 Woonton et al.

METHOD FOR MAKING FOUNDRY MOULDS Inventors: Kent Woonton; Jeffrey R. Short, 1]]; Don Mills; Kenneth N. Roach, all of Toronto, Canada Assignee: J. R. Short Milling Company, 7

Chicago, Ill.

Filed: June 21, 1971 Appl. No.: 154,763

US. Cl 164/18, 164/40, 164/210 Int. Cl. B22c 25/00, B220 15/02 Field of Search 164/17, 18, 27, 37,

References Cited UNITED STATES PATENTS 8/1911 Howard 164/40 SAND BINDER CONTINUOUS MIXER 7 11 June 19, 1973 FOREIGN PATENTS OR APPLICATIONS 603,754 8/1960 Canada 164/40 4,417,125 7/1969 Japan 164/207 Primary Examiner.1. Spencer Overholser Assistant Examiner-John E. Roe'thel Attorney-Roylance, Abrams, Berdo & Kaul Method for continuous, high speed production of foundry mould parts by pressing them from a continuous bed of foundry sand mix which is based on a cur able binder, pressing of the part being carried out at a time when the mix has adequate plastic flowability for pressure moulding and adequate potential curability to assure a strong, rigid cured product.

ABSTRACT 15 Claims, 3 Drawing Figures CURING OVEN FINISHED MOULD STORAGE PAIEIIIEIIII I 3.739.834

SAND BINDER l: l'

6 CONTINUOUS MIXER 7 U CURING OVEN FIG. 1 FINISHED 2 MOULD f STORAGE IILT t FIG. 2 3

I. IIIIIII.

I2 I II '9 I7 20 I IO 20 T I8 I I I I I I5 II I I4 IIIIII IIILIB EIIIII METHOD FOR MAKING FOUNDRY MOULDS BACKGROUND OF THE INVENTION In present foundry practice, moulds are made in a number of ways, including the production of green sand moulds and the production of shell moulds. However, despite long acceptance and continuous refinement, including in recent years various approaches designed to automate such practices, the practices of the prior-art have had a number of draw backs, and these have become more critical in recent years because of the need for improving working conditions in the foundries and the need to make moulds more rapidly with a lower manpower requirement.

Much of the requirement for foundry moulds has heretofore been met by making green sand moulds, following procedures usually requiring use of sea coal and providing a mould which is not cured in the sense that it has adequate strength to allow extensive handling. The use of sea coal has always been objectionable because that material is dirty, contributing a great part, if not the primary part, of the dirt common in foundries today. The weak, uncured nature of green sand moulds has also been a seriously limiting factor, even in the recently developed automatic moulding systems, since such moulds cannot be handled extensively or transported in the usual sense, as by trucks and the like, and the green sand moulds therefore must be made at the location where they are to be used. The practice of using green sand moulds has other disadvantages, including limitations on the dimensional accuracy of the moulds, the need for metal moulding flasks to prevent metal run-out, a relatively high time and labor requirement in mould production, excessive raw material storage requirements, including storage for sea coal, and excessive sand bum-on during casting, with attendant excessive time requirements in the cleaning room.

While shell mould practices offer some advantages, including elimination of sea coal and the production of moulds which are cured and therefore have adequate strength to withstand considerable handling, shell moulds have only a relatively limited applicability and cannot be adopted as an extensive replacement for green sand moulds and are subject to the disadvantages of high pattern time and cost and the tendency for the shell mould itself to warp.

OBJECTS OF THE INVENTION A general object of the invention is to provide a method for high rate, automated production of fully cured foundry moulds from dry sand, avoiding use'of sea coal and like objectionable ingredients, with the moulds having greater dimensional accuracy and being sufficiently strong to withstand handling and transportation.

Another object is to'provide a method for producing foundry moulds of such nature that the castings made therewith carry less sand and require shorter and easier operations in the cleaning room than is the case with castings from green sand moulds.

SUMMARY OF THE INVENTION Generally stated, the method of the invention comprises preparing a mix including dry sand and a curable binder material; establishing a bed of this mix on a supporting surface at a forming station; forcing a pattern into the supported bed at the forming station to press the mix into the shape desired for the cope or drag of the mould, and at the same time separating the pressed mould copy or drag from the uncompressed mix; and curing the pressed mould part. The mix employed is of such plastic nature as to be mouldable by pressing under acceptable conditions of pressure and time, and is also characterized by curability after being shaped by the pressing operation. Support of the bed of mouldable sand mix is typically accomplished by a conveyor belt, with a supporting element backing up the belt at the forming station, and with the shaping press and belt operated at speeds to give, e.g., a production rate of up to 20 mould parts per minute.

In order that the invention can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, wherein:

FIG. 1 is a semi-diagrammatic illustration of the method;

FIG. 2 is a bottom plan view of a pressing die useful in carrying out the method; and

FIG. 3 is a sectional view taken generally on line 3--3, FIG. 2.

GENERAL DESCRIPTION OF THE INVENTION In practicing the method, the initial composition or sand mix must be mouldable under conditions of reasonable pressure and time, and must retain this charac teristic for a period of time adequate to allow the mix to be prepared, the bed to be formed, and the bed to be advanced to the pressing station. Mouldability can be characterized as requiring both plastic flowability, so that the mix will respond to the pressing operation, and early toughness or tenacity, to assure that the pressed part will retain its shape as an integral body with precise dimensions and surfaces. In addition, the mix must be capable of being cured under reasonable conditions of time, e.g., preferably not exceeding 15 min. and in all events not more than min., and temperature, e.g., from room temperature to 600 F. It is also necessary that the binder material employed be of such nature that the cured binder material will be destroyed or burned out during casting of metal in the mould, and that the entire mix be of such nature that evolution of nitrogen and hydrogen be minimized when the mould is in contact with the liquid metal during casting.

A wide variety of binder materials can be employed, including the alkali metal silicate binders, the curable polymeric materials, core oil binders, and mixed binder systems, e.g., systems comprising both a curable polymer and a core oil, and systems comprising both a sili-- cate and a polymeric material. Certain binder materials are especially advantageous because they provide both lysts are included, or on binder materials cured solely by application of heat or by gassing the pressed mould part with a gaseous curing agent or agents. In all instances, the formulation is such that, at the time pressing is to be accomplished, the mix has adequate plastic flowability to be pressed to shape, an adequate interparticle plastic adhesion to assure that the shape imparted during pressing will be retained precisely, and enough residual cure capability to assure that, after final curing, the mould part will have adequate strength for handling and adequate hot strength for casting. Accordingly, in any application of the method, the time period between completion of mixing and presentation of the bed or layer at the pressing station must be selected, in view of the cure rate which characterizes the binder material employed, so that, at the time the mould part is pressed, progressive curing will not have proceeded so far as to create a relatively rigid interparticle bond which would be disrupted by the pressing step and then could not be re-established by further curing of the binder material.

Curable polymeric materials useful according to the invention are the thermoset resins, including the phenolic resins, the urea-furan resins, the water soluble modified resorcinol resins, furfuryl alcoholformaldehyde resins, and the isocyanate resin systems including, in addition to the isocyanate, a hydroxylcontaining coreactant, e.g., a hydroxyl-containing drying oil. Polymeric materials based on an aromatic polyisocyanate, an oil-modified alkyd resin and a suitable catalyst or catalysts are particularly effective. Such materials are described, for example, in U. S. Pat. No. 3,255,500, issuedJune 14, 1965, to James J. Engel and Vernon L. Guyer, and US. Pat. No. 3,426,831, issued Feb. 11, 1969, to Janis Robins and Robert J. Schafer.

Suitable alkali metal silicate binder materials are those based on aqueous solutions or dispersions containing at least 10 percent by weight of the alkali metal silicate and having a silica to alkali metal oxide weight ratio in the range of 0.5:1 5:1. Sodium silicate solutions in which the SiO :Na O weight ratio is 1:1 3.5:1 are particularly useful. The alkali metal silicate content of the aqueous material can be as high as 65 percent by weight, with contents of at least 30 percent by weight being particularly effective. Mixes based on alkali metal silicate binder materials can be cured by evaporation of water, by application of heat, by gassing with an acid gas such as C00 or by including in the binder material a curing catalyst such as an aqueous acid solution or a latent acid catalyst, e.g., a glycerol mono-, di-, or tri-acetate. Conventional additives can be included, such as the organosilicones or alumina employed to improve collapsibility or shake-out characteristics; the alkali metal siliconates and like agents used to improve resistance to moisture; and kerosene, for both lubricating and improving moisture resistance.

With all binder materials according to the invention, it is desirable to minimize the amount of binder in order to reduce the raw materials cost. One way in which this is advantageously accomplished is by use of an inexpensive compatible extender for the primary binder material. Particularly useful extenders for the polymeric binders are the so-called CTLA hydrocarbon polymer oils, prepared generally as described in U. S. Pat. No. 2,861,966, issued Nov. 25, 1958, to Joseph L. Betts and John P. Thorn. Such polymer oils have a Staudinger molecular weight of ZOO-1,000, an iodine number of 240320, and a boiling point in the range of 400-1,000 F. Also useful as extenders are other core oils prepared by combining polymerized unsaturated hydrocarbons and, e.g., a drying oil.

When the binder employed is one which does not in itself provide adequate early toughness, additional additive materials are employed for this purpose. Such materials include cereal products, e.g., maize (corn) flours and such flours which have been partially dextrinized; wood flour; fire clay; china clay; bentonite; fine sand; bank sand; silica flour; and iron oxide.

In selecting formulations for use according to the invention, care must be taken to assure that the sand mix will retain its mouldability for a period of time adequate to accomplish the manipulative steps necessary to press the mould part and, particularly, to assure that curing of the binder does not progress so far, before pressing is done, that the interparticle bond will be defeated by the pressing step. One way to determine suit ability of a sand mix for use according to the invention is to measure the change in density of standard rammed test specimens over that period of time required for mixing the materials, delivering the mix and forming the bed, presenting the bed at the pressing station, and carrying out pressing. That time period will of course vary, depending upon, e.g., apparatus details. Assuming the use of continuous mixing apparatus for production of the sand mix, and an endless belt apparatus on which the bed is formed and which advances the bed to the pressing station, typical time periods from start of mixing to completion of pressing can be on the order of 220 mins., and for illustrative purposes can be taken as 10 mins. in testing sand mixes for suitability, cyclindrical specimens 2 inches long and 2 inches in diameter, rammed three times with a 14 lb. weight, according to AFS Standard Foundry Sand Mixture Test Specimen, pages 4-4 through 4-l 1 of the Foundry Sand Handbook, Seventh Ed., 1963, published by the American Foundrymens Society, can be employed. Density comparisons are made between a specimen from a first sample, rammed immediately after mixing is completed, and a speciment prepared from a second sample, rammed at a given time (e.g., 6 min., when the mixing time is 4 min., to give the 10 min. test period) after mixing is completed. A density variation between the two specimens not exceeding 6 percent, and advantageously not exceeding 4 percent, for most compositions employing a polymeric binder material or an alkali metal silicate binder material, and 14 percent for oil sand mixes, indicates that the composition is suitable according to the invention.

When progressively curable polymeric binder materials are employed, suitability according to the foregoing test procedure can be achieved with proportions of binder material ranging up to 10 percent of the weight of sand employed. Particularly satisfactory results are obtained with binder materials comprising both an amount of a polyisocyanate-oil modified alkyd resin combination equal to 0.5-1 percent of the sand weight and an amount of CTLA hydrocarbon polymer oil correspondingly equal to 0.5-0 percent of the sand weight.

When an aqueous alkali metal silicate binder material is employed, suitability according to the foregoing test procedure can be achieved with proportions of the binder material equal to 2-6 percent of the weight of the sand in the mix.

The following examples illustrate typical sand mix formulations useful according to the invention.

EXAMPLE 1 ingredient The resin employed was an isocyanate-oil modified alkyd resin system supplied by Reichhold Chemicals (Canada) Ltd, Weston, Ontario, as product 44-718,

and the catalyst and drier were from the same supplier under the respective designations 44-716 and 92-076.

Mixing was in a Simpson Muller, using a 2 3 mixing cycle.

EXAMPLE 2 Parts by Weight Ingredient Mix C Mix D Sand 4000 4000 Partially dextrinized maize flour 40 40 Modified resorcinol, water solution 60 60 Catalyst 8 8 Water 60 100 Simpson Muller, using a mixing cycle of l-4-2.

EXAMPLE 3 Parts by Weight Mix G 4000 Ingredient Sand Iron oxide 20 Fire clay 20 Furfurylated aminoaldehyde resin 80 Catalyst 24 Mix F 4000 Mix H 4000 20 The furfurylated aminoaldehyde resin was that supplied by Reichhold Chemicals (Canada) Limited under product designation 21-315, and the catalyst was 75 percent phosphoric acid.

EXAMPLE 4 Parts by Weight Mix K 4000 40 40 120 40 40 ingredient Sand iron Oxide Partially dextrinized maize flour Water Core oil Western bentonite Mix L 4000 The oil employed was a glyceride oil supplied by Archer-Daniels-Midland Co., Minneapolis, Minnesota, U.S.A., under the trademark LINOIL 250. Mixing was carried out in a Simpson Muller with a cycle of l-4-2.

EXAMPLE 5 Parts by Weight Mix M Mix N Sand 4000 4000 Alkyd isocyanate resin 32 CTLA polymer oil 8 Catalyst 6.4 Drier 2.4

Ingredient The resin, catalyst, drier and mixing cycle were as in Example 1. The CTLA polymer oil was that supplied by Imperial Oil Ltd., Toronto, Ontario Canada.

EXAMPLE 6 Parts by Weight Ingredient Mix P Mix Q Mix R Sand 4000 4000 4000 Aqueous sodium silicate I20 120 Liquid acidic catalyst 19.2 4.8

The aqueous sodium silicate employed was that supplied by Philadelphia Quartz Co., Philadelphia, Pennsylvania, U. S. A., under the product designation RU, having an SiO :Na O weight ratio of 2.4:1, an Na O content of 13.85 percent by weight, an SiO content of 33.2 percent by weight, a density of 52 Be., and a viscosity of 2,100 cps. The catalyst was that supplied by Ashland Oil & Refining Co., Ashland, Kentucky, U. S. A., under the product identification Catalyst 3005. The mixtures were prepared in a Simpson Muller with cycle 3 for Mix P, and a 2+3 cycle for mixes Q and R.

Referring to FIG. 1, the ingredients for the sand mix are supplied to a suitable continuous mixing apparatus and the mix so formed is discharged onto an endless horizontal conveyor belt 1 which is driven stepwise to move the upper run of the belt to the right, as viewed. Parallel side plates 2 are provided, one extending along each side of the belt, in such fashion that the upper run of the belt and the side plates cooperate to form a trough efiective to retain the sand mix. A horizontal scraper 3 extends between the side plates and is spaced above the upper run of the belt 1 to form the sand mix into a continuous bed 4 of predetermined depth as the upper run of the belt advances.

In a location spaced from scraper 3 in the direction of movement of the upper run of the belt, a flat stationary back-up plate 5 is rigidly mounted beneath the upper run of the conveyor in a position such that the conveyor belt slides smoothly over the back-up plate in flush contact therewith. In this location, there is mounted above the conveyor a vertically acting rectilinear power device, such as the conventional pressure operated motor 6 of the piston and cylinder type. A die, indicated generally at 7 and described hereinafter in detail, is rigidly mounted on the end of the piston rod 8 of motor 6. Operation of motor 6 to drive the piston downwardly is effective to move: die 7 downwardly, from an initial position spaced above the level of bed 4, until the die has pressed the increment of bed 4 therebelow into the shape of the desired mould part and has severed the mould part from the trailing portion of the bed.

Operation of conveyor 1 is carried out in timed relation to cyclic operation of motor 6 so that, when die 7 is in its raised position, the conveyor presents a fresh increment of bed 4 and, while pressing is carried out, the belt and therefore the bed are stationary. Once a mould part has been pressed and the die withdrawn upwardly, the next cycle of movement of conveyor 1 carries the shaped mould part away from the pressing station, the mould parts ultimately being delivered toa continuously operated curing oven.

An important feature of the invention is that the pressing operation is carried out in such fashion that each stroke of the pressing die, cooperating with the conveyor belt and back-up plate, forms the corre sponding portion of bed 4 into a complete mould part,

by what is in essence a pressure moulding action, and also severs that mould part from the trailing bed. To accomplish this, a die having the construction seen in FIGS. 2 and 3 is employed. The die comprises a die plate having a flat upper face to the center of which is welded an internally threaded bushing 11 for rigid connection of the die plate to the piston rod 12. The opposite face of the die plate presents the male pattern 13 to be pressed into the sand mix bed 4.

The plan shape of plate 10 is rectangular and the die plate is slidably embraced by a rectangular confining and severing shroud 14. Shroud 14 comprises four flat side members 15 welded together in rectangular fashion, the inner face of each member 15 being in flush slidable engagement with a different one of the edges of plate 10. Side members 15 lie in vertical planes and the bottom edge of each side member is formed as a knife edge, at 16. Welded to the upper portions of two opposite side members 15, so as to be above the die plate, are two bars 17 each provided near its respective ends with two plain upright bores 18, so that there is one bore 18 near each corner of the assembly. Bores 18 freely accommodate, respectively, four screws 19, the threaded ends of which are engaged in upwardly opening bores in die plate 10. Compression springs are provided, each surrounding a different one of screws 19 and engaged between the head of the screw and the upper face of the corresponding bar 17.

When the die is raised out of engagement with bed 4, springs 20 urge screws 19, and therefore die plate 10, upwardly so that the upper face of the die plate engages the lower faces of bars 17. The pattern 13 is therefore spaced a significant distance above the lower cutting edges 16 of side members 15. When the die is driven downwardly into bed 4, the side members 15 are forced completely through the bed, stopping only when engaged with the portion of belt 1 supported by plate 5. Movement of the piston rod downwardly still continues, so that die plate 10 is forced against the bed 4 with pattern 13 creating the desired moulded impression. Such downward movement of the combination of the piston rod can be limited by a stop or stops (not shown) provided on the piston rod or on screws 19. When the piston rod again moves upwardly to withdraw the die to its inactive position, the mould part M is left on belt 1, completely severed from the bed 4, the cavity portion, sides and top of the mould part being completely pressure formed by the action of the die.

Considering FIG. 2, it will be seen that two opposite side members 15 of the die lie in vertical planes which extend along the inner v faces of the respective side plates 2 of the apparatus, so that these sides of the die are essentially in sliding contact with side plates 2 when the die engages bed 4. Thus, for practical purposes, once the shroud 14 is forced downwardly into contact with belt 1, shroud l4 cooperates with belt 1 and die plate 10 to completely confine substantially the entire increment of bed 1 disposed at the forming station. The action at the forming station thus comprises severing that increment from the trailing portion of the bed, confining the severed increment of the bed, and then completing the pressing operation by further movement of the die plate, with upward withdrawal of the die then leaving the shaped mould part as an independent article ready to be moved away from the pressing location by the next step of movement of the conveyor belt.

Excellent results are obtained with most mix formulations according to the invention when the forming operation is carried out with a die plate pressure, that is,

the pressure applied to the sand mix bed 4 by plate 10,

on the order of 30 psi, with pressures in the range of l0-40 p.s.i., being useful.

It will be understood that springs 30 can be replaced by individual fluid pressure operated power devices.

Using a laboratory scale apparatus constructed according to FIG. 1, but with the mix prepared in the Simpson Muller supplied to belt 1 by hand, excellent mould parts were formed with Mixes A and B of Example 1, using a die plate pressure of 29.5 p.s.i., carrying out the pressing step within 5 min. after completion of the mixing cycle, and curing the pressed mould parts for times as short as 30 min. at room temperature. Similarly, mould parts are successfully formed with the same apparatus, using the Mixes identified as C, D and E in Example 2, with a die plate pressure of 29.5 p.s.i. and curing for times ranging from as short as 10 min. to as long as 50 min. at 425F. With similar mixes, cure times as low as 2.5 min. at 425F. have been attained.

When the binder employed comprises a polymeric material cured with the aid of a catalyst, or a catalyst and drier, the relative proportions of catalyst or catalyst and drier can be adjusted to control the mouldability of the sand mix to correspond to the time allowed, between mixing and pressing, by the production rate of the apparatus employed. Thus, for example, when the apparatus is operated at a high production rate, with pressing of the mould part occuring very soon after mixing, the core rate for Mixes A and B of Example 1 can be accelerated by increasing the amount of drier employed to, e.g., 20-30 parts by weight. Similarly, when the nature of the apparatus employed is such that a greater time must be expended between mixing and pressing, the cure rate for the polymeric binders can be slowed by use of a CTLA polymer oil. Thus, Mix M of Example 5 gave best results when the mould part was pressed 5 minutes after mulling was completed, and Mix 0 gave best results when the mould part. was pressed 10-15 minutes after mulling.

We claim:

1. The method for producing foundry mould parts, comprising preparing a foundry sand mix comprising a binder material which can be progressively cured to render the mould part rigid; depositing said mix on a movable supporting surface in the form of a bed of predetermined depth; moving the supporting surface to a forming station to present the bed at the forming station at a time when the mix is characterized by adequate plastic flowability for forming and the binder material retains a curing potential adequate for rigidification of the formed mould part; forcing a pattern into the bed while the bed is supported on the supporting surface at the forming station and while the mix is still characterized by adequate plastic flowability for forming and thereby pressing the mix into the shape desired for the mould part; moving the supporting surface away from the forming station to remove the mould part therefrom as an independent article; and curing the mould part to render the same adequately strong for handling and transport.

2. The method according to claim 1, wherein the mix is deposited continuously on a conveying surface in a location spaced from the forming station and is advanced as a continuous bed to the forming station; and

I time of mixing.

4. The method according to claim 3, wherein the binder material comprises an isocyanate resin and a hydroxyl-containing coreactant.

5. The method according to claim 4, wherein the coreactant is an oil-modified alkyd resin.

6. The method according to claim 3, wherein the binder material is present in the mix in an amount not exceeding percent of the sand weight and comprises a primary polymeric binder material and a substantial proportion of a hydrocarbon polymer oil having a Staudinger molecular weight of 200-l ,000, an iodine number of 240- 320, and a boiling point in the range of 400-1,000 F.

7. The method according to claim 6, wherein the primary polymeric binding material comprises an isocyanate resin and an oil-modified alkyd resin.

8. The method according to claim 3, wherein the step of forcing a pattern into the mix is accomplished when an AF S Standard Foundry Sand Mix Test Specimen prepared therefrom will exhibit a density not differing more than 4 percent from that of a like specimen prepared at the time of mixing.

9. The method according to claim 3, wherein the curable binder material is an aqueous alkali metal silicate material having an alkali metal silicate content of at least 10 percent by weight, the silica to alkali metal oxide weight ratio being 0.5:1 5:1.

10. The method according to claim 9, wherein the curable binder material is an aqueous sodium silicate material containing at least 30 percent by weight sodium silicate and having an Si0 :Na O weight ratio of at least 2:1, the amount of said binder material being equal to 2-6 percent of the weight of sand employed in the mix.

11. The method according to claim 10, wherein the mix contains a minor proportion of an acidic liquid curing catalysts for the sodium silicate binder material.

12. The method according to claim 9, wherein the mould part is thermally cured.

13. The method according to claim 9, wherein at least part of the step of curing the mould part is accomplished by gassing the same with an acidic gas.

14. The method according to claim 2, wherein the conveying surface is presented by an endless conveyor and the continuous bed is confined between surfaces spaced apart transversely of the conveyor.

15. The method according to claim 2, wherein the conveying surface is advanced stepwise to present a new portion of the continuous bed at the forming station after each mould part has been formed.

UNITED STATES PATENT OFFICE OERTIFICATE OF CORRECTION PZACEDC NO 0 3 a y Dated June Inventor(s) Kent Woonton, Jeffrey R. Short, III, Kenneth N. Roach It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 3 should read Inventors: Kent Woonton, of Kitchener, Canada;- Jeffrey R.

Short, III, and Kenneth N. Roach, both of Toronto, Canada.

Signedand sealed this -18th day of December'1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JRQ RENE D. TEGTMEYER Attesting Officer Acting Commissioner of 'Patents 

2. The method according to claim 1, wherein the mix is deposited continuously on a conveying surface in a location spaced from the forming station and is advanced as a continuous bed to the forming station; and the step of forcing the pattern into the bed severs the mould part from the bed.
 3. The method according to claim 2, wherein the curable binder material is selected from the group consisting of curable polymeric binder materials and alkali metal silicate binder materials and the step of forcing a pattern into the mix is accomplished when an AFS Standard Foundry Sand Mix Text Specimen prepared therefrom will exhibit a density not differing more than 6 percent from that of a like specimen prepared at the time of mixing.
 4. The method according to claim 3, wherein the binder material comprises an isocyanate resin and a hydroxyl-containing coreactant.
 5. The method according to claim 4, wherein the coreactant is an oil-modified alkyd resin.
 6. The method according to claim 3, wherein the binder material is present in the mix in an amount not exceeding 10 percent of the sand weight and comprises a primary polymeric binder material and a substantial proportion of a hydrocarbon polymer oil having a Staudinger molecular weight of 200-1,000, an iodine number of 240-320, and a boiling point in the range of 400*-1,000* F.
 7. The method according to claim 6, wherein the primary polymeric binding material comprises an isocyanate resin and an oil-modified alkyd resin. The method according to claim 3, wherein the step of forcing a pattern into the mix is accomplished when an AFS Standard Foundry Sand Mix Test Specimen prepared therefrom will exhibit a density not differing more than 4 percent from that of a like specimen prepared at the time of mixing.
 9. The method according to claim 3, wherein the curable binder material is an aqueous alkali metal silicate material having an alkali metal silicate content of at least 10 percent by weight, the silica to alkali metal oxide weight ratio being 0.5:1 - 5:1.
 10. The method according to claim 9, wherein the curable binder material is an aqueous sodium silicate material containing at least 30 percent by weight sodium silicate and having an SiO2: Na2O weight ratio of at least 2:1, the amount of said binder material being equal to 2-6 percent of the weight of sand employed in the mix.
 11. The method according to claim 10, wherein the mix contains a minor proportion of an acidic liquid curing catalysts for the sodium silicate binder material.
 12. The method according to claim 9, wherein the mould part is thermally cured.
 13. The method according to claim 9, wherein at least part of the step of curing the mould part is accomplished by gassing the same with an acidic gas.
 14. The method according to claim 2, wherein the conveying surface is presented by an endless conveyor and the continuous bed is confined between surfaces spaced apart transversely of the conveyor.
 15. The method according to claim 2, wherein the conveying surface is advanced stepwise to present a new portion of the continuous bed at the forming station after each mouLd part has been formed. 